Gas sensor with conductive housing portions

ABSTRACT

A gas sensor assembly having a first housing portion having a receptacle formed therein and a second housing portion in the form of a cover. A gas-sensing agent is disposed in the receptacle, and a working electrode, a counter electrode, and a reference electrode provided on a single electrode support sheet are disposed in fluid contact with the gas-sensing agent. The first housing portion is provided with three conductive housing portions which are in electrical contact with the three electrodes, and the cover maintains pressurized contact between the three conductive housing potions and the three corresponding electrodes. The gas sensor may be provided with two different types of leakage detectors to sense leakage of the gas-sensing agent from the sensor.

BACKGROUND OF THE INVENTION

The present invention is directed to a gas sensor used to detect thepresence of gases, such as carbon monoxide.

Most commercially available gas sensors are of the amperometric typehaving two or more electrodes in which a catalytically active metal isfixed to a porous substrate. The porous substrate operates as a gaspermeable membrane. The electrodes are located on the inside surface ofthe membrane where they are immersed in an electrolyte such as sulfuricacid. External circuitry maintains one of the electrodes, the workingelectrode, at a given electrical potential with respect to one of theother electrodes.

When the gas of interest diffuses through the porous membrane to reachthe working electrode, the diffused gas is oxidized or reduced at theinterface of the working electrode and the electrolyte. That reactiongenerates an electrical current that is proportional to theconcentration of the gas. In some cases, the gas of interest reacts withanother chemical which, in turn, is oxidized or reduced. In some cases,sensors are of a galvanic design wherein a metal such as lead isoxidized to provide the potential at the working electrode.

In the prior art, the sensors were connected to the external circuitthrough wires. For example, a platinum contact wire was connected to thecatalytically active electrode and passed through the sensor body to anexternal contact. Since most sensors contain a corrosive, liquidelectrolyte, a difficulty with sensors has been providing secureelectrical contact with the electrodes while maintaining a seal at thelocation where the conductor passes through the sensor body. In theprior art, seals around conductors have included Teflon gaskets. Inother methods, the seal has been made of thermoplastic material or epoxyresin.

One difficulty with such prior art sensors has been that such seals weredifficult and expensive to make. Accordingly, there was a need in theprior art for an improved sensor design that would avoid the need forsuch seals and thereby be more reliable and less expensive tomanufacture. Also, there was a need for a sensor that was mechanicallystronger and more durable than sensors known in the prior art whereinthe electrodes were connected to platinum wires.

Previous carbon monoxide sensors are relatively complicated in designand suffer from a number of disadvantages due to the use of sulfuricacid, which is typically used as the electrolyte. These disadvantagesinclude the fact that the sulfuric acid may leak from the sensor and/orcause the internal components of the sensor to corrode.

SUMMARY OF THE INVENTION

The present invention is directed to a gas sensor, such as a carbonmonoxide sensor, which is relatively simple in structure and inexpensiveto manufacture.

In one aspect, the invention is directed to a gas sensor assembly havinga housing with a receptacle formed therein, a gas-sensing agent disposedin the receptacle, and a plurality of electrodes disposed in fluidcontact with the gas-sensing agent. The housing has a plurality ofconductive housing portions, each of which is conductively coupled to arespective one of the electrodes, and a non-conductive housing portionconductively separating the conductive housing portions. By providingthe conductive housing portions instead of the relatively thin wirestypically used to connect the gas sensor electrodes to a conventionalgas sensing circuit, assembly of the gas sensor is simplified and a morerugged construction is obtained.

In another aspect, the invention is directed to a gas sensor assemblyhaving a housing with a receptacle formed therein, a gas-sensing agentdisposed in the receptacle, and a plurality of electrodes disposed influid contact with the gas-sensing agent. The gas sensor has a pluralityof conductive members, one for each electrode, and means for maintainingpressurized contact between the conductive members and the electrodes.The means for maintaining pressurized contact, which preferably providesa pressure of at least about 20 pounds per square inch between theelectrodes and the conductive members, may comprise a second housingportion, such as a cover, and a rubber gasket disposed between thehousing portion and the second housing portion. The gas sensor mayinclude a single electrode support sheet, disposed between the rubbergasket and the housing portion, having the electrodes formed thereon.

In a further aspect, the invention is directed to a gas sensor assemblyhaving a housing with a receptacle formed therein, a gas-sensing agentdisposed in the receptacle, a plurality of electrodes disposed in fluidcontact with the gas-sensing agent, and a plurality of conductivemembers, each of which is disposed in electrical contact with arespective one of the electrodes. A closure member is disposed adjacentthe housing in which the receptacle is formed, and a liquid-tight sealis formed between the closure member and the housing forming thereceptacle so that the closure member and the housing form an enclosedliquid-tight chamber. None of the electrodes and none of the conductivemembers pass through the liquid-tight seal, thus ensuring that theintegrity of the seal is not compromised. The closure member maycomprise an electrode support sheet on which the electrodes are formed.

In another aspect, the invention is directed to a gas sensor assemblyhaving a housing with a receptacle formed therein, a gas-sensing agentdisposed in the receptacle, and a plurality of electrodes disposed influid contact with the gas-sensing agent. The gas sensor includes aleakage detector, disposed in the housing, which generates a visualindication of leakage upon the gas-sensing agent coming into contactwith the leakage detector. The leakage detector may comprise asubstantially flat sheet impregnated with a material which changes colorupon coming into contact with the gas-sensing agent, and the flat sheetmay be disposed directly beneath a transparent portion of the gas sensorhousing so that the color change is readily apparent.

In another aspect, the invention is directed to a gas sensor assemblyhaving a housing with a receptacle formed therein, a gas-sensing agentdisposed in the receptacle, and a plurality of electrodes disposed influid contact with the gas-sensing agent. The gas sensor includes aleakage detector in the form of a pair of spaced conductive membersassociated with the housing, the spaced conductive members beingprovided adjacent the receptacle. The spaced conductive members have arelatively high resistance between them in the absence of gas-sensingagent coming into contact with them and a relatively low resistancebetween them in the presence of gas-sensing agent coming into contactwith them. The change in resistance due to gas-sensing agent leakingfrom the receptacle and coming into contact with the spaced conductivemembers can be detected.

These and other features and advantages of the present invention will beapparent to those of ordinary skill in the art in view of the detaileddescription of the preferred embodiment, which is made with reference tothe drawings, a brief description of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a preferred embodiment of a gassensor in accordance with the invention;

FIG. 2 is an illustration of an electrode support sheet used in thesensor of FIG. 1;

FIGS. 3-6 are various views of a housing portion of the sensor of FIG. 1in which a receptacle for the storage of a gas-sensing agent is formed;

FIGS. 7A and 7B illustrate the formation of a heat seal utilized in thesensor of FIG. 1; and

FIGS. 8 and 9 illustrate an alternative embodiment of a gas sensor inaccordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of an electrochemical sensor 10 in accordancewith the invention is illustrated in FIG. 1. The sensor 10 has a housingcomposed of a first, generally cup-shaped housing portion 12 (see alsoFIG. 4) in which a receptacle 14 for the storage of an electrolyte orgas-sensing agent is formed and a second housing portion in the form ofa snap-fit cover 16 which is retained on the cup-shaped housing portion12 via an annular rib 18, integrally formed with the cover 16, whichrests ina matching annular groove formed in the housing portion 12.

A flat circular electrode support element, in the form of a sheet 20, arubber gasket 22, and a leakage detector in the form of aleakage-indicating sheet 24 are disposed between the housing portion 12and the cover 16. The rubber gasket 22 and the sheet 24 are flat andcircular and each have a central hole formed therein in which a smallcylindrical filter 26 is disposed. The leakage-indicating sheet 24 maybe composed of a paper-like substrate, such as Paper No. BSF-65commercially available from Whatman Specialty Co., impregnated with aconventional agent, such as dimethyl yellow, which changes color (inthis case to red) when the gas-sensing agent comes into contact with it.The change in colorcan be viewed through the cover 16, which is formedof a transparent material.

The cover 16 has an opening or sensing hole 28 formed therein directlyabove the filter 26 so as to expose the filter 26 to the ambientatmosphere to be sensed by the gas sensor 10. The purpose of the filter26, which may be a rubber charcoal filter, is to prevent certain gases(which are not to be sensed) that may interfere with the sensing of thedesired gas(es) from passing into the interior of the sensor 10 wherethe gas-sensing reaction takes place. The electrode support sheet 20 ishydrophobic to prevent the liquid gas-sensing agent from escaping fromthesensor 10 via the sensing hole 28 but allows passage there through ofthe gaseous atmosphere to be sensed.

The gas sensor 10 includes a wick 30, which may be composed of glasspaper,for example. The wick 30 includes a first portion, shownhorizontally in FIG. 1, which abuts the underside of the electrodesupport sheet 20 and several portions which extend downwardly into thegas-sensing agent disposed in the receptacle 14. The purpose of the wick30 is to maintain the underside of the electrode support sheet 20 (whichhas three electrodes formed thereon) in fluid contact with thegas-sensing agent. Where the gas sensor 10 is used to detect thepresence of carbon monoxide,the gas-sensing agent may be a 30% sulfuricacid gel.

FIG. 3 is a top view of the cup-shaped housing portion 12 having thereceptacle 14 formed therein. The cup-shaped housing 12 has threeconductive housing portions 40, 42, 44, each of which serves as anelectrical conductor. The three conductive portions 40, 42, 44 areelectrically isolated from each other by the remaining non-conductiveportions 46 of the housing 12. The conductive portion 40 is composed ofthree portions: an arcuate upper portion 40a having a relatively smallradial width, a side portion 40b having a relatively large radial width,and a bottom portion 40c, all three of which are shown in FIG. 4. Thesideportion 40b acts as a side contact surface at which an electricalconnection can be made, and the bottom portion 40c acts as a bottomcontact surface at which an electrical connection can be made. Theshapes of the conductive portions 42, 44 are similar to that of theconductive portion 40 in that they each have an arcuate upper portion(e.g. portion 44a) 42a, 44a having a relatively small radial width, aside portion (e.g.portion 44b) 42b, 44b having a relatively large radialwidth, and a bottom portion (e.g. portion 44c) 42c, 44c.

It is not necessary that the conductive housing portions 40, 42, 44 haveboth side and bottom portions; the provision of both side and bottomportions simply increases the area to which electrical connection may bemade. The shape of the conductive portions 40, 42, 44 may be varied fromthose shown in FIGS. 3-6.

The electrode support sheet 20, the bottom surface of which is shown inFIG. 2, has three conductive electrode patterns 50, 52, 54 formedthereon.The pattern 50, which acts as a working electrode, includes afirst centralportion 50c, a second portion 50a, and a third portion 50bwhich electrically interconnects the portions 50a, 50c. The pattern 52,which acts as a reference electrode, and the pattern 54, which acts as acounterelectrode, are similarly composed of three conductive portions52a, 52b, 52c, 54a, 54b, 54c. The electrode patterns 50, 52, 54 maycomprise platinum powder disposed on a Teflon substrate in aconventional manner.

When the electrode support sheet 20 is placed on top of the housingportion12, the conductive portions 50a, 52a, 54a of the electrodesupport sheet 20are aligned and make contact with the arcuate conductiveportions 40a, 42a,44a of the housing 12. The electrode support sheet 20may be provided with an alignment mechanism, such as a tab (not shown),to ensure that the conductive portions 50a, 52a, 54a are accuratelyaligned with the conductive portions 40a, 42a, 44a.

In the manufacture of the gas sensor 10, the cup-shaped housing 12 isformed via a conventional dual-injection molding process described asfollows. First, the housing 12 without the three conductive portions 40,42, 44 is injection-molded in a first mold with a non-conductiveplastic, such as polypropylene. The result of the first mold will be ahousing portion 12 as shown in FIGS. 3-6, but with air being presentwhere the conductive portions 40, 42, 44 are shown. The housing portion12 is then placed in a second mold, and the conductive portions 40, 42,44 are injection-molded with a conductive plastic, such as polypropylenehaving carbon or other conductive fragments melted therein. The resultof this conventional dual-molding process is the housing 12 shown inFIGS. 3-6 in which the non-conductive portions 46 and the conductiveportions 40, 42, 44 together form a unitary construction.

After the cup-shaped housing 12 is formed, a suitable gas-sensing agentis disposed within the receptacle 14, and then the wick 30 is disposedover the gas-sensing agent. The receptacle 14 is then sealed via aheat-sealingprocess to give it a liquid-fight seal. Referring to FIG.7A, a portion of the sidewall of the cup-shaped housing 12 and a portionof the electrode support sheet 20 are shown enlarged. The cup-shapedhousing 12 has an upper surface 60 and a meltable member 62 having anupper surface which isat a greater elevation than the surface 60. Duringthe heat-sealing process, the electrode support sheet 20 is forceddownwards over the top of the cup-shaped housing 12 by a flat heatedplaten (e.g. 210° C.)for a predetermined period of time (e.g. fiveseconds). As a result, the meltable member 62 will melt, as shown inFIG. 7B, to form a heat-sealed bond with the electrode support sheet 20.It should be understood that since the overall shape of the meltablemember 62 is circular and since the circular member 62 surrounds theentire outer periphery of the cup-shaped housing 12, a heat-sealed bondis formed about the entire periphery of the housing 12 so that thegas-sensing agent is completely confined within the receptacle 14.

To facilitate ease of manufacture, the gas-sensing agent may beintroduced into the housing 12, after the heat seal is formed asdescribed above, through a hole or tube in the bottom of the housing 12(with the housing 12 being inverted). After the gas-sensing agent isadded, the hole or tubeis permanently closed (e.g. in the case of ameltable tube, by melting the tube closed).

One advantage of the manner of making the heat seal described above isthatthe Teflon material of the electrode support element 20 is bondeddirectly to the polypropylene plastic of the cup-shaped housing 12, withno conductive potions 40, 42, 44 or 50, 52, 54 coming into contact withthe seal. That is advantageous because the heat-sealing of Teflon andpolypropylene forms a relatively strong bond, while the presence ofconductive portions, such as those formed by platinum powder, may resultin a weaker bond.

After the heat-sealed bond is formed, the rubber gasket 22, theleakage-indicating sheet 24, and the filter 26 are placed on theelectrodesupport sheet 20, and the cover 16 is snap-fit over theassembly and is retained in place by the annular rib 18. The verticallocation of the rib 18 with respect to the underside of the cover 16 isdimensioned so that, when the cover 16 snap-fit onto the housing portion12, the underside of the cover 16 exerts a pressure of at least about 20psi, and preferably about 25 psi, on the electrode support sheet 20,thus ensuring that the conductive portions 50a, 52a, 54a of theelectrode support sheet 20 are always in electrical contact with theconductive portions 40a, 42a, 44a ofthe housing 12.

In operation, to detect a gas, a constant voltage is placed between theworking electrode 50c and the reference electrode 52c via the conductivecontact portions 40c and 42c(which are electrically connected to theelectrodes 50c, 52c, respectively). Then, upon the presence of the gasbeing detected through the sensing hole 28, an electrical current willbe induced between the working electrode 50c and the counter electrode54c, which current can be detected and measured by a conventionalcurrent sensing circuit attached to the conductive contact portions 40cand 44c (which are electrically connected to the electrodes 50c and54c).

A second embodiment of the gas sensor 10 is illustrated in FIGS. 8 and9. Referring to those figures, the cup-shaped housing 12 is modified bythe addition of two conductive housing portions 70, 72 which togetheract as asecond type of leakage detector. In the event that anygas-sensing agent leaks from the receptacle 14 and bridges the gapbetween the two conductive housing portions 70, 72, the electricalresistance between those two portions 70, 72 will change from arelatively large value (due to the non-conductive housing portionseparating the portions 70, 72) to arelatively small value (since thegas-sensing agent has a relatively low electrical resistance). Thissignificant change in resistance can be detected in a conventionalmanner by a conventional detecting circuit connected to both of theconductive portions 70, 72. The conductive portions 70, 72 are formed inthe same manner as the conductive portions 40, 42, 44 formed during thedual-injection molding process described above.

Additional modifications and alternative embodiments of the inventionwill be apparent to those skilled in the art in view of the foregoingdescription. This description is to be construed as illustrative only,andis for the purpose of teaching those skilled in the art the best modeof carrying out the invention. The details of the structure and methodmay bevaried substantially without departing from the spirit of theinvention, and the exclusive use of all modifications which come withinthe scope of the appended claims is reserved.

What is claimed is:
 1. A gas sensor assembly comprising:a housing havinga receptacle formed therein; a gas-sensing agent disposed in saidreceptacle; and a plurality of electrodes disposed in fluid contact withsaid gas-sensing agent. said housing having a plurality of conductivehousing portions, each of said conductive housing portions beingconductively coupled to a respective one of said electrodes, saidhousing having a non-conductive housing portion with a plurality ofspaces formed therein, said conductive housing portions being disposedwithin said spaces, wherein said conductive housing portions arecomposed of a conductive plastic material.
 2. A gas sensor assemblycomprising:a housing having a receptacle formed therein; a gas-sensingagent disposed in said receptacle; and a plurality of electrodesdisposed in fluid contact with said gas-sensing agent, said housinghaving a plurality of conductive housing portions, each of saidconductive housing portions being conductively coupled to a respectiveone of said electrodes, said housing having a non-conductive housingportion with a plurality of spaces formed therein, said conductivehousing portions being disposed within said spaces, wherein said housinghas a contact surface that is formed by a portion of said non-conductivehousing portion and a portion of each of said conductive housingportions, wherein said gas sensor assembly additionally comprises anelectrode plate disposed adjacent said contact surface of said housing,and wherein said electrodes are disposed on said electrode plate.
 3. Agas sensor assembly comprising:a housing having a receptacle formedtherein; a gas-sensing agent disposed in said receptacle; and threeelectrodes disposed in fluid contact with said gas-sensing agent. saidhousing having a non-conductive housing portion and three conductivehousing portions, each of said three conductive housing portion beingconductively coupled to a respective one of said electrodes, and threeconductive housing portions being conductively separated from each otherby said non-conductive housing portion, wherein said conductive housingportions are composed of a conductive plastic material.
 4. A gas sensorassembly comprising:a housing having a receptacle formed therein; agas-sensing agent disposed in said receptacle; and three electrodesdisposed in fluid contact with said gas-sensing agent, said housinghaving a non-conductive housing portion and three conductive housingportions, each of said three conductive housing portions beingconductively coupled to a respective one of said electrodes, and saidthree conductive housing portions being conductively separated from eachother by said non-conductive housing portion, wherein said housing has acontact surface that is formed by a portion of said non-conductivehousing portion and a portion of each of said conductive housingportions, wherein said gas sensor assembly additionally comprises anelectrode plate disposed adjacent said contact surface of said housing,and wherein said electrodes are disposed on said electrode plate.
 5. Agas sensor assembly comprising:a housing having a receptacle formedtherein, said housing having an external surface; a gas-sensing agentdisposed in said receptacle; and three electrodes disposed in fluidcontact with said gas-sensing agent, said housing a non-conductivehousing portion and three conductive housing portions, each of saidthree conductive housing portion being conductively coupled to arespective one of said three electrodes, said external surface of saidhousing being formed by a portion of said non-conductive housing portionand a portion of each of said three conductive housing portions, whereinsaid conductive housing portions are composed of a conductive plasticmaterial.
 6. A gas sensor assembly comprising:a housing having areceptacle formed therein, said housing having an external surface; agas-sensing agent disposed in said receptacle; and three electrodesdisposed in fluid contact with said gas-sensing agent, said housinghaving a non-conductive housing portion and three conductive housingportion, each of said three conductive housing portions beingconductively coupled to a respective one of said three electrodes, saidexternal surface of said housing being formed by a portion of saidnon-conductive housing portion and a portion of each of said threeconductive housing portions, wherein said housing has a contact, surfacethat is formed by a portion of said non-conductive housing portion and aportion of each of said conductive housing portions, wherein said gassensor assembly additionally comprised an electrode plate disposedadjacent said contact surface of said housing, and wherein saidelectrodes are disposed on said electrode plate.
 7. A gas sensorassembly comprising:a housing having a receptacle formed therein, saidhousing having a contact surface; an electrode plate disposed adjacentsaid contact surface of said housing; a gas-sensing agent disposed insaid receptacle; and a plurality of electrodes disposed on saidelectrode plate and in fluid contact with said gas-sensing agent, saidhousing having a non-conductive housing portion and a plurality ofconductive housing portions, each of said conductive housing portionsbeing conductively coupled to a respective one of said electrodes, saidcontact surface of said housing being formed by a portion of saidnon-conductive housing portion and a portion of each of said conductivehousing portions.
 8. A gas sensor assembly as defined in claim 7 whereinsaid conductive housing portions are composed of a conductive plasticmaterial.
 9. A gas sensor assembly as defined in claim 7 wherein saidreceptacle comprises a cylindrical opening formed in said non-conductivehousing portion.
 10. A gas sensor assembly as defined in claim 7 whereinsaid housing has an external surface that is formed by a portion of saidnon-conductive housing portion and a portion of each of said conductivehousing portions.
 11. A gas sensor assembly comprising:a housing havinga receptacle formed therein; a gas-sensing agent disposed in saidreceptacle; and a plurality of electrodes disposed in fluid contact withsaid gas-sensing agent, said housing having a plurality of conductivehousing portion, each of said conductive housing portions beingconductively coupled to a respective one of said electrodes, saidhousing having a non-conductive housing portion conductively separatingsaid conductive housing portions, said conductive housing portions andsaid non-conductive housing portion being formed so that no portion ofany said conductive housing portions comes into direct contact with saidgas-sensing agent, wherein said conductive housing portions are composedof a conductive plastic material.
 12. A gas sensor assembly comprising:ahousing having a receptacle formed therein; a gas-sensing agent disposedin said receptacle; and a plurality of electrodes disposed in fluidcontact with said gas-sensing agent, said housing having a plurality ofconductive housing portions, each of said conductive housing portionsbeing conductively coupled to a respective one of said electrodes, saidhousing having a non-conductive housing portion conductively separatingsaid conductive housing portions, said conductive housing portions andsaid non-conductive housing portion being formed so that no portion ofany of said conductive housing portions comes into direct contact withsaid gas-sensing agent, wherein said housing has a contact surface thatis formed by a portion of said non-conductive housing portion and aportion of each of said conductive housing portions, wherein said gassensor assembly additionally comprises an electrode plate disposedadjacent said contact surface of said housing, and wherein saidelectrodes are disposed on said electrode plate.
 13. A gas sensorassembly comprising:a housing having a receptacle formed therein; agas-sensing agent disposed in said receptacle; a plurality of electrodesdisposed in fluid contact with said gas-sensing agent, said housinghaving a plurality of conductive housing portions, each of saidconductive housing portions being conductively coupled to a respectiveone of said electrodes, said housing having a non-conductive housingportion conductively separating said conductive housing portions, saidconductive housing portions and said non-conductive housing portionbeing formed so that no portion of any of said conductive housingportions comes into direct contact with said gas-sensing agent; a firsttype of leakage detector that detects the presence of gas-sensing agentleakage from said receptacle; and a second type of leakage detector thatdetects the presence of gas-sensing agent leakage from said receptacle.14. A gas sensor assembly as defined in claim 13 wherein said first typeof leakage detector comprises a substantially flat sheet impregnatedwith a material which changes color upon coming into contact with saidgas-sensing agent.